Ultrasound techniques are commonly used as non-invasive or non-destructive diagnostic tools in a range of industries including medicine, foodstuffs, pharmaceuticals, petrochemicals, chemicals and materials processing. In known ultrasound methods, ultrasound echoes are transmitted to scatterers in a medium and backscattered or scattered echoes are detected. Ultrasound parameters such as backscattering coefficient, angular scattering, attenuation, speed of sound, material nonlinearity and statistics can then be used to reveal intrinsic material properties of the scatterers or the medium such as microstructure and/or composition. In the case of ultrasound imaging of a biological tissue, the radio-frequency ultrasound signal backscattered by the components of the tissue has been used to extract quantitative properties of the scatterers to reveal tissue properties such as the correlation length of the medium. This method and the other aforementioned approaches have been used successfully to detect and diagnose medical conditions, such as prostate cancer, early Duchenne muscular dystrophy, cell apoptosis and carcinomas.
However, these methods are not suitable for all applications. They are particularly unsuitable for characterizing dense concentrations of scatterers in a medium such as dense suspensions of particles. It is desirable to obtain quantitative information regarding the physical dimensions of such scatterers. For example, in two-phase systems such as solid particles/droplets of insoluble liquid/gas bubbles in a fluid, it is desirable to quantitatively characterize the suspensions in terms of the suspended particle size, concentration and other physical parameters. One such application is in medical diagnostics where the aggregation of red blood cells is known to be an independent risk factor of circulatory related disorders such as thrombosis, atherosclerosis and valvular heart disease. Also, the presence and size of embolisms in the blood vessels of a patient can be indicative of their risk of suffering a stroke. In industrial and food processing applications, particle size and shape characterizations are important in the quality control of many manufacturing processes such as slip casting, catalytic processes, fermentation processes, paper and paint manufacturing, as well as in the monitoring of emulsions and colloids and wear and failure of equipment parts.
In many applications such as medical ultrasound imaging, food processing and non destructive testing, the medium to monitor is composed of many weak scatterers. In pulse-echo mode, these scatterers are weak in the sense that the echoes they generate have a small amplitude in comparison to the transmitted sound wave, i.e. their acoustic parameters are closed from the acoustic parameters of their surrounding medium. For weak scatterers in a relatively dilute medium, the power of the backscattered signal increases with increasing scatterer concentration and size. This linear relationship has been exploited to monitor relative increases or decreases in the scatterer size and concentration. However, these known ultrasound techniques cannot provide quantitative or absolute physical parameters or accurate qualitative estimates of the physical parameters of the scatterers in dense medium.
The quantitative characterization of scatterers by ultrasound is further compounded by signal loss due to attenuation by intervening material between the scatterers being characterized and an ultrasound probe. Attenuation can be particularly problematic in the characterization of biological material because of the frequency-dependent attenuation due to intervening tissue layers that distorts the frequency dependence of scattering from the tissue microstructure.
In known attenuation compensation methods, the backscatter coefficient is compensated a posteriori with known values of attenuation based on the estimated thicknesses of the intervening attenuating layers. However, this method can result in inaccurate compensation, especially in the case of characterization of biological material as intervening tissue layer attenuations can vary between layers and can also vary between patients. It is also known to first evaluate the attenuation of the intervening tissue layers and then compensate a posteriori the backscatter coefficient with this attenuation value. However, the attenuations of each intervening layer between the ultrasound probe and the site of the scatterers being characterized must be estimated by a separate measurement technique which means that the compensation cannot be in real-time or performed by a single device. Furthermore, these methods assume strong hypotheses on spatial tissue homogeneity which can lead to inaccuracies.
A further limitation of known prior art ultrasound scatterer characterization techniques is that some of them rely on sampling a bulk suspension to be tested (i.e. “in vitro” methods). However, these in vitro methods cannot provide real-time analysis as samples from a bulk suspension must be isolated to be analyzed. This is undesirable in the case of manufacturing processes where the process must be interrupted to take a sample, or where the substance concerned is toxic, caustic, hot or pressurized. Sampling can also introduce contamination into the bulk or can dilute the bulk. Also, it can introduce sampling errors if the sample taken is not indicative of the bulk. In the case of medical applications, such as detecting red blood cells or other particulates in blood samples, there is the obvious drawback of having to take a blood sample from the patient and the associated health and safety issues for both the patient and the person taking the blood. In addition, it is believed that the microstructure of blood, particularly the aggregation of red blood cells, varies within the vasculature according to flow conditions and local release of substances promoting aggregation. It is therefore preferable to measure in situ and in vivo, the state of aggregation of the red blood cells, as well as other scatterers.
Therefore, there is a need for an improved method and system for ultrasound scatterer characterization.